The total purchase price should be the first thing written.
The down payment price should then be written under the purchase price.
Subtract the down payment amount from the purchase price amount.
This will leave you and your client with a balance amount due.
Also, address all terms and conditions regarding your sale...in other words, additional payment due dates for balance, with or without penalties, late fees, etc.
Address, also, any interest that may or may not incur over a period of time.